Field of the Invention
The invention relates to a process for continuously preparing an alcohol mixture, in which an input mixture which comprises an olefin and has a composition that changes over time is subjected to an oligomerization to obtain an oligomerizate and at least a portion of the olefin oligomers present in the oligomerizate are hydroformylated with carbon monoxide and hydrogen in a hydroformylation in the presence of a homogeneous catalyst system to give an aldehyde, at least some of which is converted to the alcohol mixture by subsequent hydrogenation.
Discussion of the Background
Hydroformylation—also called the oxo process—enables reaction of olefins (alkenes) with synthesis gas (mixture of carbon monoxide and hydrogen) to give aldehydes. The aldehydes obtained then correspondingly have one carbon atom more than the olefins used. Subsequent hydrogenation of the aldehydes gives rise to alcohols, which are also called “oxo alcohols” because of their genesis.
In principle, all olefins are amenable to hydroformylation, but in practice the substrates used in the hydroformylation are usually those olefins having two to 20 carbon atoms. Since oxo process alcohols can be used in a variety of ways—for instance for the production of plasticizers for PVC, as detergents in washing compositions and as odorants—hydroformylation is practiced on the industrial scale.
A good overview of the general state of hydroformylation of olefins can be found in    B. CORNILS, W. A. HERRMANN, “Applied Homogeneous Catalysis with Organometallic Compounds”, vol. 1 & 2, VCH, Weinheim, N.Y., 1996and in    R. Franke, D. Selent, A. Börner, “Applied Hydroformylation”, Chem. Rev., 2012 (112), p. 5675-5732, DOI:10.1021/cr3001803.
One example of an oxo process alcohol for which there is a high global demand is isononanol, INA for short. Isononanol is a mixture of isomeric nonyl alcohols, for example n-nonanol, and singly and/or multiply branched nonanols, such as methyloctanol in particular. INA has the CAS No. 27458-94-2 and is used essentially in plasticizer production. The C9 oxo process alcohol INA is obtained by hydroformylation of C8 olefins, for example 1-octene, to the corresponding C9 aldehydes and the subsequent hydrogenation thereof.
The patent literature contains detailed process descriptions for preparation of INA: For instance, DE102008007080A1 and EP2220017B1 disclose Co-based processes for preparing INA. EP1674441B1 discloses a two-stage INA process in which a Co-catalyzed hydroformylation is followed by an Rh-catalyzed oxo reaction.
Important criteria for distinction of industrial hydroformylation processes, as well as the substrate used, are the catalyst system, the phase division in the reactor and the technique for discharge of the reaction products from the reactor. A further aspect of industrial relevance is the number of reaction stages conducted.
In industry, either cobalt- or rhodium-based catalyst systems are used, the latter being complexed with organophosphorus ligands such as phosphine, phosphite, phosphoramidite or phosphonite ligands, in each case with trivalent phosphorus. These catalyst systems are present in the reaction mixture in homogeneously dissolved form.
The type of catalyst system and the optimal reaction conditions for the hydroformylation are dependent on the reactivity of the olefin used. The different reactivity of isomeric C8 olefins is described in:    B. L. Haymore, A. van Hasselt, R. Beck, Annals of the New York Acad. Sci., 415, 1983, p. 159-175
The products of the hydroformylation are determined by the structure of the input olefins, the catalyst system and the reaction conditions. If the aim is to obtain alcohols as conversion products of the oxo process for the production of detergents and plasticizers, the oxo reaction should produce very substantially linear aldehydes. The products synthesized therefrom have particularly advantageous properties, for example low viscosities of the resulting plasticizers.
The structure of the input olefins is highly dependent on their origin. Thus, different C8 olefins are useful for preparation of C9 aldehydes: In the simplest case, 1-octene is used, which can be prepared, for example, by oligomerization of ethylene (see below) or alongside other olefins by a Fischer-Tropsch process. 1-Octene obtained in another way is always accompanied by a large number of structurally isomeric C8 olefins. From this point of view, it is not the case that a single olefin is hydroformylated; instead, an input mixture comprising a multitude of isomeric olefins is used.
A mixture of C8 olefins having a defined number of isomers can be obtained by the oligomerization of C2 or C4 olefins:
Oligomerization is understood to mean the reaction of hydrocarbons with themselves, forming correspondingly longer-chain hydrocarbons. For example, the oligomerization of two olefins having three carbon atoms (dimerization) can form an olefin having six carbon atoms. If, in contrast, three olefins having three carbon atoms are joined to one another (trimerization), the result is an olefin having nine carbon atoms.
If butenes—i.e. what are called C4 olefins having four carbon atoms—are subjected to an oligomerization, the result is essentially olefins having eight carbon atoms (often called “dibutenes”), olefins having twelve carbon atoms (C12 olefins, “tributenes”) and, to a smaller extent, olefins having more than twelve carbon atoms (C12+ olefins). For industrial purposes, a distinction is made between what are called di-n-butenes, i.e. isomeric C8 olefins which are prepared from mixtures of 1-butene and/or 2-butenes, and what are called diisobutenes, which are obtained by dimerization of isobutene and have a higher level of branching.
Di-n-butenes, i.e. mixtures of C8 olefins which result from oligomerization of linear butenes, are of much better suitability for the preparation of highly linear oxo process alcohols than diisobutenes, since they have a much lower level of branching.
An even lower level of branching is possessed by C8 olefin mixtures which are obtained by oligomerization of the C2 olefin ethene. Ethene forms in the cracking of naphtha, but is required in large volumes for production of polyethylene and is consequently a comparatively costly raw material. Consequently, the preparation of C8 olefins from ethene is not always economically viable, even though high-value oxo process alcohols can be produced therefrom.
Much less expensive than ethene are the C4 olefin mixtures which likewise form in the cracking of naphtha (called crack-C4). The preparation of C9 alcohols from these raw materials by the route of the oligomerization and subsequent hydroformylation and hydrogenation is of economic interest if it is possible to prepare sufficiently linear isononanol from an inexpensive C4 mixture of inferior quality.
Another process practiced in industry for oligomerization of C4 olefins is called the “OCTOL process”. Detailed description thereof can be found in the non-patent literature, for example in:    B. Scholz: The HÜLS OCTOL Process: Heterogeneously catalyzed dimerization of n-butenes and other olefins. DGMK-Tagung [Meeting of the German Society for Petroleum and Coal Science and Technology] in Karlsruhe, published in Erdol, Erdgas, Kohle, April 1989, pages 21 and 22.    R. H. Friedlander, D. J. Ward, F. Obenaus, F. Nierlich, J. Neumeister: Make plasticizer olefins via n-butene dimerization. Hydrocarbon Processing, February 1986, pages 31 to 33.    F. Nierlich: Oligomerize for better gasoline. Hydrocarbon Processing, February 1992, pages 45 to 46.
Within the patent literature, DE102008007081A1, for example, describes an oligomerization based on the OCTOL process. EP1029839A1 is concerned with the fractionation of the C8 olefins formed in the OCTOL process.
The OCTOL process is generally conducted in a plurality of stages with the aid of a reactor cascade comprising a number of series-connected reaction zones or reactors corresponding to the number of stages. Between each of the individual reaction zones is provided a distillation column which removes the oligomers previously formed from the unconverted butenes in the oligomerizate and discharges them. The unconverted butenes are partly recycled into the upstream oligomerization, and the rest are fed to the downstream oligomerization.
A further multistage process for oligomerization of C4 olefins is known from WO99/25668 or from DE10015002A1. Here, dilution of the olefin streams provided by recycled butanes is practiced, in order to simplify the removal of heat from the exothermic reaction via the reactor output.
According to the manner in which the individual n-butene molecules are joined in the course of the oligomerization, an oligomerizate with a different level of branching is obtained. The level of branching is described by the iso index, which states the mean number of methyl groups per C8 molecule in the isomer mixture.
The iso index for dibutene is defined in formula (1):Iso index=(proportion by weight of methylheptenes+2*proportion by weight of dimethylhexenes)/100  (1)Thus, n-octenes contribute 0, methylheptenes contribute 1 and dimethylhexenes contribute 2 to the iso index of a product mixture of C8 olefins. The lower the iso index, the less branched the structure of the molecules within the mixture.
For the properties of the plasticizer, the level of branching of the olefinic starting mixture which is used for the preparation of the plasticizer alcohol plays a crucial role: the higher the linearity of the C8 olefin mixture, the better the properties of the C9 plasticizer alcohol prepared therefrom. The aim in the preparation of dibutene as starting material for plasticizer alcohols is thus to run the oligomerization so as to obtain a C8 product mixture having a minimum iso index.
For example, in EP1029839A1, the fractionation of the oligomers is set up such that the C8 product mixture removed has a minimum iso index.
WO99/25668A1 achieves a low iso index in another way, by recycling such amounts of the butane removed from the oligomer and of unconverted butene into the oligomerization that the maximum content of oligomers in the converted reaction mixture does not exceed 25% by weight anywhere in the reactor cascade.
Each process utilized a “raffinate II” having a high proportion of 1-butene as starting mixture for the oligomerization. “Raffinate II” is commonly understood to mean a butane/butene mixture which is obtained from C4 cuts which originate from steamcrackers and from which butadiene and isobutene have already been removed. For instance, typical raffinate II contains around 50% by weight of 1-butene.
It can be shown that a high proportion of 1-butene in the hydrocarbon mixture provided has a favorable effect on the linearity of the oligomerizate. It is therefore unsurprising that WO99/25668A1, proceeding from the raffinate II raw material, produces C8 product mixtures having an iso index less than 1.
Nierlich too, in his above-cited article “Oligomerize for better gasoline”, emphasizes that raffinate II is of better suitability as starting material for an oligomerization than raffinate III. “Raffinate III” is obtained by removing 1-butene from raffinate II, and for that reason has a much lower 1-butene content than raffinate II. Since 1-butene is likewise an important target product in C4 chemistry, which finds use, inter alia, in the preparation of particular polyethylene-based plastics having a low density (linear low-density polyethylene, LLDPE), there is a great industrial interest in the isolation of 1-butene from raffinate II streams. The effect of this is increasingly that increasingly only raffinate III streams are available for other chemical uses, for example oligomerization.
Because of the current shortage of raw materials as well, petrochemically produced raffinate II is no longer available everywhere in large volumes and under favorable conditions. For instance, some of the C4 olefin mixtures obtained from alternative raw material sources contain hardly any 1-butene, but contain predominantly 2-butene.
One particular example here is FCC-C4, which originates from fluid-catalytic crackers. Other low-1-butene C4 sources are chemical processes such as the dehydrogenation of butenes, and the fermentation or pyrolytic conversion of renewable raw materials.
In contrast to conventional crack-C4, these alternative C4 mixtures do not just have a low proportion of 1-butene but are also subject to variations with time in terms of their composition:
Thus, the C4 stream from an inconstant source can generally be regarded as a sum total of individual mass flows of pure substances, with variation in the respective substance mass flow rates within particular ranges of substance mass flow rates with a particular rate of variation. Table 1 shows the dynamic specification of a C4 stream having a composition which varies with time.
TABLE 1Dynamic specification of an inconstant C4 streamSubstanceSubstance mass flow rateRate of variationIsobutene:0 kg/s to 1 kg/s−0.05 g/s2 to 0.05 g/s21-butene:0 kg/s to 6 kg/s−0.30 g/s2 to 0.30 g/s22-Butene (cis + trans):1 kg/s to 13 kg/s−0.30 g/s2 to 0.30 g/s2Isobutane:0 kg/s to 3 kg/s−0.15 g/s2 to 0.15 g/s2n-Butane:1 kg/s to 7 kg/s−0.30 g/s2 to 0.30 g/s2Other materials:0 kg/s to 1 kg/s−0.05 g/s2 to 0.05 g/s2
Thus, a stream according to the specification could deliver 1 kg of 1-butene per second, and this value could vary at a rate of 0.25 g/s2. This means that, within 100 000 seconds (=28 hours), the component mass flow rate of 1-butene rises up to 3.5 kg/s. Conversely, a decrease in the 1-butene with a rate of variation of −0.1 g/s2 is also in accordance with the specification, and so a 1 kg/s 1-butene stream dries up completely within less than 3 hours (namely within 10 000 seconds), and so the C4 stream is suddenly free of 1-butene. Since all the components are subject to variations with time, there may also be variations in the total mass flow rate. These variations are limited by the limits of the operating range of the particular plant.
The preparation of high-quality oxo process alcohols from such inconstant raw material sources is the technically demanding background to this invention. In spite of the technical difficulties, there is great economic interest in such a process, since the increasing scarcity of C4-containing raw material streams means that smaller streams from many different sources will have to be used in combination in the future, in order to be able to adequately supply world-scale plants for C4 processing. These streams therefore have very different compositions, which may even vary according to the time of year in some cases. This leads to a much more inconstant raw material supply than supply with crack-C4 from one or few naphtha crackers which has been customary to date.
Both in the field of oligomerization and in that of hydroformylation, the first attempts have been made to deal with varying reactant quality:
WO 99/25668 A1 states that the skeletal isomerization of the C8 product can be affected by recycling of a portion of the reactor output into the reaction.
German patent application DE 102013212481.3, which was still unpublished at the filing date, shows that it is still possible to produce a product with comparatively good isomer distribution even from inferior low-1-butene C4 streams. Both the literature sources are in agreement that, for the production of a plasticizer that meets user expectations, it is possible to use only dibutene streams having a low level of branching as starting material for the hydroformylation. The level of branching is assessed using the iso index according to formula (1). The lower the iso index, the less branched the structure of the molecules within the mixture. DE 102013212481.3 states that preferably only C8 olefin mixtures having an iso index of less than 1.10 are usable for further processing to give plasticizer alcohols. Particular preference is even given to using only mixtures having an iso index of less than 1.05.
When rhodium complexes are used as catalyst for hydroformylation, the ligand is another crucial factor for the product composition of the aldehydes. Unmodified rhodium-carbonyl complexes catalyze the hydroformylation of olefins having terminal and internal double bonds, where the olefins may also be branched, to give aldehydes having a high level of branching. The proportion of terminally hydroformylated olefin is usually lower compared to the cobalt-catalyzed product.
A ligand-modified catalyst system consisting of rhodium and triorganophosphine, e.g. triphenylphosphine, hydroformylates only olefins having a terminal double bond with high selectivity. There is barely any occurrence of isomerization of the double bond and hydroformylation of the internal double bonds.
The hydroformylation of olefins having internal double bonds over catalyst systems containing sterically demanding bisphosphite ligands, in the case of long-chain olefins, proceeds with good selectivity but not with satisfactory activity. In this regard, see:    P. W. N. M. van Leeuwen, in Rhodium Catalyzed Hydroformylation, P. W. N. M. van Leeuwen, C. Claver (eds.), Kluwer, Dordrecht, 2000.
Rhodium-monophosphite complexes in catalytically active compositions, in contrast, are suitable for the hydroformylation of branched long-chain olefins having internal double bonds.
Since the 1970s, there have been descriptions of the use of “bulky phosphites” in hydroformylation:    H. Tricas, O. Diebolt, P. W. N. M. van Leeuwen, Journal of Catalysis, 2013 (298), p. 198-205
These feature good activity, but the n/i selectivity for terminally hydroformylated compounds is in need of improvement.
As well as the use of pure ligands, the use of ligand mixtures has also been described in the literature.
US 20120253080 describes the use of monophosphites with bisphosphites. They are used as a “monitoring ligand”. Bisphosphites have much higher complex formation constant than monophosphites and thus form more stable but less active catalyst complexes. However, this combination has the disadvantage that, although the bisphosphites have excellent selectivity, their activity in the case of long-chain olefins is in need of improvement, since not only the selectivity for the desired product but also the space-time yield and the activity of the catalyst system play a crucial role in an industrial scale process. Moreover, the bisphosphites are usually much more costly to prepare than, for example, monophosphites.
EP1099678B1 describes the combined use of phosphonites with bisphosphites. However, it is disadvantageous here is that both ligand types are very costly to produce, and an industrial scale process can therefore hardly be economically viable. Moreover, the addition of the bisphosphite ligand noticeably affects the yield of the reaction, since these ligands are less active when dibutene, for example, is used as substrate. A further problem is that phosphonites are also very much less stable than other organophosphorus ligands.
According to the process chosen, the discharge from the dibutene hydroformylation contains a mixture of the isomeric C9 aldehydes (called isononyl aldehydes, INAL for short) and the isomeric C9 alcohols (called isononyl alcohols, INA for short). The mixture arises from the fact that hydroformylation catalysts are also hydrogenation-active to a certain degree, meaning that they catalyze the reaction of the aldehydes with the hydrogen present in the synthesis gas to give alcohols. Since only the isononyl alcohols are used for the envisaged application, the discharge from the hydroformylation is subjected to a hydrogenation in order to convert the aldehydes present to alcohols. The catalytic hydrogenation of aldehydes to alcohols has been broadly described in the literature; it may proceed either with heterogeneous catalysis or with homogeneous catalysis. An overview is given by    J. Falbe, H. Bahrmann, W. Lipps, D. Mayer, G. D. Frey, Alcohols Aliphatic in Ullmann's Encyclopedia of Industrial Chemistry, 2013and    D. Sanfilippo, P. N. Rylander, Hydrogenation and Dehydrogenation in Ullmann's Encyclopedia of Industrial Chemistry, 2009.
The hydrogenation process does not have any measurable influence on the isomer distribution, meaning that the skeletal isomers of the aldehyde are found in the same ratio in the alcohol.
EP1430014B1 discloses that the isomer distribution of the isononyl alcohol directly affects the properties of the plasticizer produced therefrom by esterification. Esterification with carboxylic acids, especially with phthalic acid, is likewise well-known. In industry, the alcohol mixture is esterified with phthalic acid or phthalic anhydride. The product used as plasticizer is then the diisononyl phthalate (DINP). This can in principle also be prepared by transesterification of a dialkyl phthalate with isononyl alcohol; in this context, dimethyl phthalate in particular is used. EP1430014B1 shows that the process step of esterification has no influence on the quality of the plasticizer; this is determined exclusively by the properties of the C9 alcohol.
There are qualitative rules for the dependence of the performance properties on the structure. Wilson, for example, discusses the properties of phthalates as a function of the total carbon number, meaning the molar mass, and the level of branching, i.e. the isomer composition among other factors:    Alan S. Wilson, Plasticizers, The Institute of Materials, 1995, ISBN 0 901716 76 6, pages 135-136.
Given the same molar mass, for example, the effects of increasing branching include the adverse effects of increasing viscosity, rising vapor pressure, which is associated with a higher volatility, lower plasticizing action and lower thermal and light stability.
Conversely, high branching also brings about positive effects, namely better PVC compatibility, lower migration, greater hydrolysis resistance, lower biodegradation (during the use phase) and higher electrical resistance.
It is immediately apparent that there is no best plasticizer; a compromise has to be made according to the use. Thus, if the plasticizing effect is the most important, a plasticizer having minimum branching will be preferable. If, in contrast, the aim is to produce cable sheathing with PVC, products having somewhat higher branching if anything will be chosen because the electrical insulating action is better.
Using some synthetically prepared individual isomers of DINP, Wadey et al. show very clearly how important properties depend on the structure of the esters:    Brian L. Wadey, Lucien Thil, Mo A. Khuddus, Hans Reich; The Nonyl Phthalate Ester and Its Use in flexible PVC, Journal of Vinyl Technology, 1990, 12, p. 208-211
In summary, it can be stated that the performance properties depend on the structure of the phthalate, the latter depends fundamentally on the structure of the alcohol, and the latter in turn depends considerably on the structure of the parent olefin. An additional complicating factor is that these compounds are produced as isomer mixtures.
Once a formulation has been developed, it then has to be ensured in production that a product having uniform properties is produced. For example, in the case of lubricants, viscosity can vary only within tight limits, in order to comply with the desired viscosity class. In the case of plasticizers, for example, the plasticizing action must remain constant, in order not to force the processors of the plasticizer into constant formulation adjustment, or the electrical resistance must not be below a limit. Constant production monitoring through performance tests is, however, completely impossible for reasons of time, quite apart from the costs that this would cause.
Keeping the isomer composition of the input mixture constant could theoretically solve the problem, but this cannot be sustained in practice. A cracker is operated not in order to give a C4 stream of constant composition, but in order to produce ethylene, propylene, gasoline or other mass products. In practice, the composition of the C4 stream available from the cracker will always vary according to the composition of its raw material and according to the mode of operation. The problem becomes more serious when C4 streams are bought in from different crackers, as is nowadays common practice in large-scale production and will ever more frequently be the case in the future.
Merely by virtue of the raw material at the start, there will therefore inevitably be changes in the composition of the product, and hence changes in the isomer distribution and therefore changes in the performance properties. As already discussed above, further changes in the product composition come to rise in the downstream steps of oligomerization and hydroformylation, for example through alteration of the operating conditions or ageing of the oligomerization catalyst.
According to all of the above, it can be stated that the problem of constant product quality has to date not been highlighted in a holistic manner. The individual process steps have always been considered as independent units. The quality of the plasticizer produced by conventional processes varies within defined limits, this variation being random and dependent on numerous outside influences.